Add configuration options for Buzz Browser class, ...

Add configuration options for Buzz Browser class, and addressed issue of hard-wired call to file_get_contents() that shouldn't be.

I added this because Buzz Browser was unable to retrieve the thumbnails for videos that were remote in the sandbox application, and I traced it down to not being able to configure the proxy, which I am behind at work.

While I was at it, I added options for configuring all the public setters in the Buzz Browser AbstractClient.php class. In sonata_media.yml, you might add something like this:

sonata_media:
    buzz:
        client:
            proxy: %sonata_media.buzz.client.proxy%,

And in parameters.yml, you might add something like this:

parameters:
    sonata_media.buzz.client.proxy: tcp://example.com:80

Please consider integrating the changes in this pull request, thanks!